x86/boot/doc: Fix grammar and typo in boot.txt

Fixes some minor issues in the x86 boot documentation.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Rob Landley <rob@landley.net>
Link: http://lkml.kernel.org/r/20121026031702.GA23828@www.outflux.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>